Nickel-silicon thermocouple element

ABSTRACT

A THERMOELEMENT NICKEL-SILICON ALLOY WHICH IS INTERCHANGEABLE WITH THE ALUMEL ELEMENT IN A CHROMEL-ALUMEL THERMOCOUPLE, THE NEW ALLOY HAVING A GREATER RESISTANCE TO OXIDATION AND A GREATER EMF STABILITY THAN ALUMEL.

F. s.s|B|.EY ETAL 3,535,083

NICKEL-SILICON THERMocouPLE ELEMENT 3 Sheets-Sheet 1 4v BAvnD 4d 'lawnlv woud Noun/m30 June l5, 1971 Filed April 21. 1969 3 Sheets-Sheet 2 F. S. SIBLEY ETAI- NlcxEL-SILICON THERMocoUr-LE ELEMENT o Emana 1d IBWOUHS wozu Noam/m30 R lo motqamaml.. Zorcqmu mm ooo. oom o mo d I. M W w 1 m1 wm N5 G I. JTD 3 F r Arilfnlv A m N /q/OJ Onu L A N m lill! /w w M m //.A l l DWV V. E L m u o m 2u E w .EEUMTQ o M S l M lmoyr u F es, D vw n Y .1 a o.. m. f\ aou w 3. .\vm u A N .0E -nl June l5, 1971 Filed April 21, 1969 /gzrm Ma/SLM ATTORNEYS June 1'5, 1911 F. S. SIBLEY ETAL NICKEL-SILICON THERMOCOUPLE ELEMENT Filed April 2l, 1969 3 Sheets-Sheet 3 M KM %MMM ATTORNEYS United States Patent O 3,585,083 NICKEL-SILICON THERMOCOUPLE ELEMENT Forbes S. Sibley, Troy, and Norman F. Spooner, Bloomlield Township, Oakland County, Mich., assignors to Hoskins Manufacturing Company, Detroit, Mich.

Filed Apr. 21, 1969, Ser. No. 817,703 Int. Cl. H01v 1/22 U.S. Cl. 136-236 8 Claims ABSTRACT OF THE DISCLOSURE A thermoelement nickel-silicon alloy which is interchangeable with the Alumel element in a Chromel-Alumel thermocouple, the new alloy having a greater resistance to oxidation and a greater EMF stability than Alumel.

This invention relates to Chromel-Alumel thermocouples and more particularly to a new alloy interchangeable with the Alumel thermoelement in a Chromel-Alumel thermocouple.

Chromel-Alumel thermocouples have been used for many years for a majority of temperature measurement applications in the range of 1500 F., to 2000 F. The popularity of this particular thermocouple can be attributed to its high EMF output, its relatively high resistance to oxidation, its relatively high EMF stability and relatively low cost. However, experience has shown that at temperatures above 1600 F. the Alumel leg oxidizes rather rapidly and loses its EMF stability with a resultant substantial change in EMF output of the thermocouple.

Various alloys have been proposed heretofore as substitutes for the Alumel element of a Chromel-Alumel thermocouple. While some of these prior art alloys have exhibited improved qualities in certain respects and under limited temperature ranges, no prior art alloys are interchangeable with the Alumel element to provide a. thermocouple which produces an EMF/temperature curve falling within the accepted tolerances for thermocouples of this type throughout a temperature range of 32 F. to 2000 o F.

It is an object of this invention to provide an alloy interchangeable with the Alumel thermoelement in a Chromel-Alumel type thermocouple which has much greater oxidation resistance and greater EMF stability than Alumel, particularly above 1600 F., and which, when used with a conventional Chromel-P element, provides a thermocouple that produces an EMF/ temperature curve which meets the accepted standard tolerances for such thermocouples at all temperatures from 32 F. to 2000 F.

In the drawings:

FIG. l illustrates EMF/temperature curves produced by several sample thermoelements of the new alloy and by several prior art thermoelements versus platinum.

FIG. 2 illustrates the EMF /temperature curve produced by a Chromel-P thermoelement versus platinum which meets the standard requirements.

FIG. 3 illustrates EMF temperature curves for thermocouples formed by combining the Chromel-P element of FIG. 2 with the live thermoelements of FIG. 1.

The alloy of the present invention which is interchangeable with an Alumel thermoelement has the following nominal composition: .20% to .50% Fe, 2.25% to 2.75%

ICC

Cu, 2.30% to 2.70% Si, .90% to 1.30% Co and the balance Ni. The preferred composition of the alloy of this invention is as follows: .20% to .35% Fe, 2.30% to 2.50% Cu, 2.40% to 2.60% Si, 1.00% to 1.20% Co and the balance Ni.

'In producing the alloy of this invention nickel and copper are melted in a coreless induction furnace under a suitable slag and are then treated with one or more deoxidants, such as alkaline earth and rare earth metals or other conventional deoxidants. The total of al1 such deoxidizing elements should not be greater than .5% and preferably should be less than .3% by weight. Iron, cobalt and silicon are added to the melt followed by a final deoXidiZation. The temperature is adjusted for pouring and the metal is cast in suitable iron molds. The conditioned ingots are hot rolled to a size suitable for cold processing and are cold drawn to finish sizes with intermittent annealing.

In the alloy of this invention the silicon is maintained at a relatively high value since it contributes to the oxidation resistance of the alloy. The silicon content can be varied within a narrow range to control the EMF of the thermoelement. Iron and copper are added to the alloy primarily for their characteristic effects on the EMF/ temperature curve; that is, in order to conform the curve of the thermoelement to within the tolerances specified by the Instrument Society of America (ISA) for Type K thermocouples such as Chromel-Alumel thermocouples. Copper also has the effect of increasing the electrical resistivity of the alloy. Cobalt has a significant effect on the EMF of the alloy and is employed in the composition primarily for this purpose. It also increases the resistivity of the alloy.

In the Table I below several typical compositions of the alloy of this invention are set forth as well as several prior art alloys heretofore suggested for use as substitutes for Alumel.

Composition, weight percent Fe Cu Si Co Ni 24 2. 33 2. 52 1. 12 Balance. 29 2. 32 2. 61 1.00 Do. 21 2. 34 2. 48 1. 30 Do. 27 2. 33 2. 55 1. 20 Do. 29 2. 39 2. 49 1.19 Do. 23 2. 34 2. 43 0. 94 Do. 20 01 2. 26 15 D0. 03 2. 23 18 Do.1 06 03 2. 38 10 Do. 28 27 2.36 40 Do.

In Table I, compositions A through F comprise the improved alloy of this invention while compositions G through J designate prior art alloys of somewhat similar composition which have been used or suggested for use as substitutes for Alumel in a Chromel-Alumel thermocouple. The various compositions set forth in Table I indicate that the new alloy differs in composition from the prior art alloys primarily in the substitution of higher contents of copper and cobalt. The new alloy also has a somewhat higher silicon content.

Chromel-Alumel thermocouples are classified by ISA as Type K thermocouples. The EMF/temperature values for Type K thermocouples accepted in industry and as published by the National Bureau of Standards are set forth in Table 1I:

TABLE II The limits of error for Type K thermocouples and eX- tension wires are specified by ISA as i4 F. between 32 F. and 530 F. and 134% of the temperature being measured between 530 F. and 2300 F. The special limits specified by ISA for Type K thermocouples are half the above values; namely 2 F. between 32 F. and 530 F. and i3/4'70 of the temperature being measured between 530 F. und 2300 F. The standard limits for Type K thermocouples are shown graphically in FIG. 3 and the half limits are shown graphically in FIG. 2. The industry accepted tolerances for single thermoelements for use in Type K thermocouples are one-half the standard limits specied for Type K thermocouples. These tolerances as well as the half limits for single thermoelements are shown graphically in FIG. 1.

Table III below sets forth the standard and half limits of Type K thermocouples at various temperatures in millivolts. It also sets forth in millivolts the accepted limits for single thermoelements as distinguished from the thermocouple itself. In addition, there is set forth in Table III the actually calibrated EMF deviations obtained with several thermoelements of the new alloy and the prior art alloys referred to in Table I.

2000 F. elements A and B fall within the half limits specified for one wire. lt will also be noted from FIG. l that the prior art thermoelements G and H produce EMF/ temperature curves which extend substantially beyond half limits specified for Type K thermocouples in the range between 32 F. and 350 F.

In order to compare the EMF/temperature curves of Type K thermocouples employing the new alloy and the prior art alloys as the electro-negative elements, reference is now made to FIGS. 2 and 3. In FIG. 2 there is shown the EMF/temperature curve for a Chromel-P element versus platinum. This curve is designated P in the drawing. Element P is a standard Chromel-P alloy having the following nominal composition: 9.5% Cr, .5% Fe. .5% Si and 89.5% Ni. As is shown in FIG. 2, the actual deviations in millivolts for alloy P from the curve fot` Chrome] versus platinum are:

FIG. 3 shows the calibration of thermocouples formed by combining the Chromel-P thermoelement designated P with each of the ve negative thermoelements shown in FIG. l. The more negative the calibration of the Chromel-P element in the range of 200 F. to 300 F. the further out of tolerance on the negative side will be the thermocouples formed by combining the Chromel-P element with the prior art negative thermoelements G and H of FIG. 1 in that temperature range. This is clearly illustrated in FIG. 3. Conversely, if the Chromel-P element were within tolerance on the positive side of the curve, the extent of out of tolerance on the negative side between 200 F. and 300 F. of the thermocouple would be less but the tolerance would still fall beyond the stand- 4" ard limits at 200 F. for any combination of alloys G and TAB LF. 111. E31 I*` LIMITS ()1 TYPE K TIIERMOCOI'I LES ANI) TYPICAL CALIB RATIUNS OI*` NEW THERMO- ELEMENT ANI) FRI() R TIIERMOELEMEINTS Elven-ical Deviation from Standard Alumel Curve, mv.l I'CSSLVily, "T 'vv-T minis/entr. 32 F. 200 F. 30 0 F. 400 F. 500 F. 1,000c F. 1.500" F. 2000 F.

1std.1imns2 0 1.080 i. 080 i. 080 1.080 :2.115 i. 208 i. 310 TIIGUHOCOUPC 011141111111115 0 i. 044 i. 044 i. 044 i. 044 i. 0s? i. 134 4.158 0 l w 1Std. limits 0 i. 044 4.044 i. 044 i. 044 i. 0147 i. 134 1.158 le 11@ --l11u1f1imi[5 0 i. 0:12 i. 022 :1;.022 1.022 i. U43 :1;.067 2.079

0 040 017 .005 .000 .020 021 .015 0 .0314 007 .004 002 033 .021 021 0 .03 .00 00 .02 .07 .07 .08 o 044 021 022 027 .02s .054 .110 0 044 021 013 025 040 .08s 142 0 .031 011 025 .030 .072 .005 .020 0 000 000 005 012 .041 004 007 0 .13 .10 07 .0s 04 .01 05 0 .11 .07 .01 .00 .03 .00 .07 0 007 000 .021 .010 05s 004 037 1 All values plus unless otherwise indicated.

A study of Table III reveals that thermoelements formed of the alloy of this invention (compositions A 60 through F) fall within the special or half limits specilied for Type K thermocouples between 32 F. and 2000 F. None of the prior art thermoelements (alloys G through J) fall within either the standard or the half limits specified for Type K thermocouples at all temperatures within this range.

The EMF/temperature curves of several thermoelements of this invention and two of the prior art alloys referred to in Table I versus platinum are shown in FIG. l. As indicated previously, FIG. 1 also shows graphically the half limits specified for Type K thermocouples and the half limits accepted for single thermoelements. Reference to FIG. 1 indicates that theromelements A, B and C of the new alloy fall within half limits of a Type K thermocouple between 32 F. and 300 F. Between 300 F. and

2 Based on Type K limits set forth above.

H with the Chromel-P element lying within the accepted limits for the Type K positive thermoelement versus platinum. FIG. 3 also shows that all combinations of compositions A, B or C with any such Chromel-P thermoelement, or any other thermoelement having EMF temperature characteristics meeting the accepted limits for Type K positive thermoelements, will lie within the Type K thermocouple limits at all temperatures from 32 F. to 2000 F.

In view of the data presented above, it is apparent that the alloy of this invention represents a decided improvement on the prior art alloys. As noted above, the prior art alloys fall outside the accepted tolerances in the temperature range of between 200 F. and 300 F. The range from 32 F. to 400 F. is particularly critical because the junctions between thermocouples and the extension wires normally operate in that temperature range.

Any mismatch in thermal EMF characteristics of a thermoelement and its corresponding extension wire at the temperature of their junction represents a built-in error in the output of the thermocouple, regardless of the temperature of the measuring junction. It is standard practice in industry to manufacture the individual thermocouple elements and corresponding extension wires to the same :L2 F. tolerance up to 400 F. The maximum error due to mismatch is therefore limited to the total tolerance band, that is 4 F., at the temperature of the thermoelement-extension wire joint, or 8 F. combined error for both thermoelements.

Since the new alloy as shown above is thermoelectrically interchangeable with Alumel within the range of 32 F. to 400 F. no loss of minimum accuracy is incurred by using the new element as a thermocouple element connected to a regular Alumel extension wire. On the other hand, a loss in thermocouple output representing at least 2 F. to 4 F. in excess of the single wire accepted tolerance would result from making such a connection at 200 F. to 300 F. between the prior art competitive alloys of Table III and an Alumel extension wire.

Alumel has a resistivity above 160 ohms per circular mil foot. Table III also shows that the improved alloy of this invention more nearly matches the electrical resistivity of Alumel than do the prior art alloys. This resistivity match enhances the compatibility of the new alloy with Alumel since it enables the interchange of Alumel and the new alloy in the same diameter wire without significantly affecting the electrical resistance of the thermocouple circuit.

In order to compare the stability of the new alloy with a standard Alumel thermoelement, samples of each were subjected to prolonged exposure at 2000 F. These samples were formed of 18 B & S gauge wires and the millivoltages produced thereby were checked after pre determined timed intervals. The results of such comparative stability tests are tabulated below in Table IV:

TABLE IV.-RELATIVE EMF STABILITY OF NEW ALLOY AND ALUMEL AT 2,000 F.

Increase in output vs. Pt, of wires aiter indicated time at 2,000 F.

50 hrs. 100 hrs. 200 hrs. 265 hrs.

Mv. F l Mv. F. Mv. F. Mv. E.

Alumel .14 6.7 .27 13 .36 17 .42 20 New alloy .04 1.9 .08 3.8 .09 4.3 .09 4.3

1 F. corresponding to each mv. value represents the temperature equivalent for a Type K thermocouple.

Alumel wire was almost twice as great as the loss in diameter of wires formed of the new alloy.

We claim:

1. A thermoelement composed of an alloy consisting essentially of .20% to .50% Fe, 2.25% to 2.75% Cu, 2.30% to 2.70% Si, .90% to 1.30% Co and the balance Ni, said thermoelement meeting the EMF/temperature values specied in Table II for the negative element of an ISA Type K thermocouple within the half limits for such thermocouple specified in Table II and as shown graphically in FIG. 1.

2. A thermoelement as called for in claim 1 composed of an alloy consisting essentially of .20% to .35% Fe, 2.30% to 2.50% Cu, 2.40% to 2.60% Si, 1.00% to 1.20% Co and the balance Ni.

3. A thermoelement as called for in claim 1 and containing not more than .5% deoxidizing elements.

4. A thermoelement as called for in claim 2 and containing not more than .3% deoxidizing elements.

5. A thermocouple meeting the EMF/temperature values for the ISA Type K thermocouple indicated in Table II Within the standard limits for such thermocouple indicated in Table III and shown graphically in FIG. 3 consisting of a nickel-chromium alloy positive thermoelement and a negative thermoelement composed of an alloy consisting essentially of .20% to .50% Fe, 2.25% to 2.75%

Cu, 2.30% to 2.70% Si, .90% to 1.30% Co and the balance Ni, said positive and negative thermoelements meeting the EMF/temperature values specified in Table II for the positive and negative elements, respectively, of the ISA Type K thermocouple each within one-half the standard limits of such thermocouple at all temperatures from 32 F. to 2000 F. as specied in Table II and as shown graphically in FIG. 1.

6. A thermocouple as called for in claim 5 wherein the negative thermoelement is composed of an alloy consisting essentially of .20% to .35% Fe, 2.30% to 2.50% Cu, 2.40% to 2.60% Si, 1.00% to 1.20% C0 and the balance Ni.

7. A thermocouple as called for in claim 5 wherein the negative thermoelement contains not more than .5 deoxidizing elements.

8. A thermocouple as called for in claim 6 wherein the negative thermoelement contains not more than .3% deoxidizing elements.

References Cited UNITED STATES PATENTS 3,457,122 7/ 1969 Sibley et al. 136--239 CARL D. QUARFORTH, Primary Examiner H. E. BEHREND, Assistant Examiner U.S. Cl. X.R. 

